add a formula for "redpen"

RedPen is a proofreading tool to help writers or programmers who write
technical documents or manuals that need to adhere to a writing
standard. RedPen automates the verifications of input documents written
in natural languages.
Can this test be modified to do something more substantial than e.g. `--version` or `--help`? See `cmake.rb` for an example of an application formula with a good test and `tinyxml2.rb` for an example of a library formula with a good test. Thanks!